Market-Based Control of Shear Structures Utilizing Magnetorheological Dampers

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Market-Based Control of Shear Structures Utilizing Magnetorheological Dampers

Michael B. Kane, Jerome P. Lynch, Member, IEEE, and Kincho Law

Abstract—The use of magnetorheological (MR) dampers for semi-active control of structures subject to seismic, wind, and/or other excitations has been an extensive field of study for over a decade. Many of the proposed feedback control laws have been based on modern linear systems control theory, e.g. linear quadratic Gaussian (LQG), H2, or H∞ control. Alternatively, this paper presents a nonlinear market-based controller (MBC) that explicitly handles the dynamic force saturation limits of MR dampers, a feature not available in the design of linear controllers. The MBC solution builds on an agent-based control (ABC) architecture with a diverse population of buying and selling agents capable of sensing and control respectively. These agents participate in a competitive market place trading control energy in a way that leads to a Pareto optimal allocation of control force during each control time step. The ABC architecture allows for easy implementation withinexpensive partially-decentralized large-scale wireless sensing and control networks. This novel controller is validated using a numerical simulation of a seismically excited six story shear structure with MR dampers at the base of V-braces installed on each story. The MBC is compared against a benchmark LQG controller in a variety of test cases including best case scenarios, robustness to agent failure, variations in ground excitation, variation in peak ground acceleration, and controller delays.

I.INTRODUCTION

C

ontrol of the dynamic response of civil structures (e.g. bridges, buildings, and towers) has been extensively studied since the 1970’s, yet a limited number of building owners are choosing structural control over more traditional passive design methodologies[1]. The high installation costs of the feedback control system components (i.e. sensors, actuators, centralized controller, and associated wiring) required to execute the control law are partially responsible. These costs can be reduced by implementing partially decentralized control architectures that utilize wireless controllers. Fortunately, the advances in low-cost microcontroller design and fabrication have recently led to the development of inexpensive wireless controllers with collocated sensing, actuation, communication, and computation abilities [2]. Lynch, Swartz, Yang, et al. have demonstrated in simulation and experimentation the ability of low cost wireless controllers,with varying degrees of decentralization,to effectively control civil structures using MR dampers[2-10].

Feedback structural control systems can be broadly grouped into two categories, active and semi-active. Active control systems (e.g. active-mass dampers) make use of actuators that directly apply forces on the structural system but utilize a substantial amount of power. Alternatively, semi-active systems indirectly apply forces on the structural system by changing member properties such as member stiffness or damping. Examples of semi-active devices include semi-active hydraulic dampers (SHD), variable-stiffness devices (VSD), variable-friction dampers, semi-active tuned mass dampers (STMD), electrorheological (ER) dampers, and magnetorheological (MR) dampers[1]. MR dampers are particularly well suited for use as semi-active devices in civil structures due to their low power requirements, small size, high dynamic range, and relatively quick dynamics[11].

Fig. 1.6-story partial-scale single-bay steel structure used for validation of MR damper control laws on the NCREE shake-table.

The design of structures utilizing V-braces as a lateral force resisting system can be slightly modified to allow for the installation of MR dampers as shown in Fig. 1. When used in this manner, the number of MR dampers in a large structure may reach into the 100s. Optimally controlling such a system with a centralized controller would require extensive lengths of cables running to a central computer. If the central computer were to fail, the entire system would enter a passive state leading to a significant drop in system performance. Alternatively, the control architecture could be completely decentralized, without the need for expensive data cabling or the linchpin of a centralized computer. However decentralized controllers cannot offer the same level of system performance when compared to centralized controllers due to each decentralized controller’s sparse information about the state of the whole system[12].

Partially decentralized control architectures can achieve a balance between centralization and decentralization. In wireless partially decentralized control, a separate controller is collocated with each actuator and utilizes its embedded sensing and computation along with a wireless transceiver to share information with other local controllers. In small structures with a reliable wireless communication environment, ‘local’ can mean all controllers in the network. However, in a large structure or for structures with an unfavorable wireless communication environment, ‘local’ can refer to only a few controllers within a reliable communication range. In order to achieve the best possible performance, the control architecture should be able to adapt to the time-varying definition of ‘local’. This paper will investigate one such control architecture that has been inspired by the control of free market economies, referred to herein as market-based control (MBC).

In this study an MBC law is formulated to control the non-linear response of a six story single bay shear structure outfitted with MR dampers subject to seismic disturbances (see Fig. 1). The control law, abbreviated MR-MBC, is specifically formulated to account for the non-linear nature of the MR dampers as described by a Bouc-Wen hysteresis model. The paper concludes with the results of a numerical simulation comparing the MR-MBC law withboth passive control and a benchmark LQG controller.

II.A Formulation Of Market Based Control For Control Of MR Dampers

The trade of goods and services has existed between humans for thousands of years. For most of that time there has been no central entity controlling the price of goods and the quantity that must be produced. Instead, each individual strives to maximize their utility with the limited amount of knowledge available to them as to how much of a good they should trade at the market price. A price is optimal to all agents in the market when there is equilibrium such that the price that buyers are willing to purchase goods at is equal to the price that suppliers are willing to manufacture enough goods to meet demand[13]. Since people have been trading for much longer then designing control systems, the theories of microeconomics are betterestablished to explain the processes behind the distributed resource allocations. The analogy between the distributed resource allocation found in economies and that found in distributed control of dynamic systems is so strong that the well established microeconomic theories are the foundation for the new field of market-based control of dynamic physical[14] and computer [15] systems.

A.Control systems as a distributed allocation problem

Controllable dynamic systems can often be described by the discrete-time state space equation presented in (1), where the system has N dynamic degrees of freedom, M inputs u(k), and time-steps k of length Δt. The state of the system can be fully described by the Nx1 state vector x(k) at each time step. The evolution of the state of the system is a function of the current state,x(k), current input, u(k), and the initial state of the system x0.

It is assumed that each controllable input is represented by an agent j that has a non-negative cost,Kj,associated with the production of each unit of production cj(k). Similarly, the nmetrics to be controlled are each represented by an agent i that receives a non-negative utility, Φi, associated with each unit of a good ui(k) received by the agent.

Therefore, the goal of the centralized resource allocation problem is the solution to the optimization problem

J is an objective function describing the efficiency of the consumption and production of a resource subject to the constraint that the sum of the resources consumed by consumers i=1…n, ui(k), is equal to the sum of the resources produced by producers j=1…M, cj(k).However due to the complex and often non-linear nature of large-scale control problems, the solution to (4) is often very difficult to find. Alternatively, Voos and Litz [14] proposed to individually maximize each agents objective function separately(5), a task that can be completed in real-time by agentsusing their onboard computing, wireless transceivers, and a set of market rules.

Equation (5), while formulated as an optimal control problem, is also in the form of a special distributed resource allocation problem that has a set of Pareto optimal resource allocations at each step in time k. Pareto optimal solutions are those solutions to resource allocation problems that occur when all consumers and producers consume (u*1(k), …, u*n(k)) and produce (c*1(k), …, c*M(k)) respectively and there is no other allocation that would increase any agents objective function without simultaneously decreasing another agent’s objective function [13].

The optimal solution to (5) can only be computed by the agents when they have been given a set of rules dictating how to trade in the market and how to maximize their objective function. First, the objective function of each agent must be formulated to map the current state of an agent to its desire to purchase or supply a quantity of a good at a particular price. Second, a set of rules must be given to each agent to optimize its objective function by trading with other agents. One set of rules could require each agent to send out its entire objective function to all other agents. Afterwards each agent solvesthe centralized resource distribution problem. If information transfer is limited, the rules could instead require each agent to send only a limited amount of information only to certain agents. In the case of limited information transfer, the convergence to the Pareto optimal allocation may require an iteration of negotiations between agents.

B.Single degree of freedom (SDOF) formulation

The goal of this paper is to present anMR-MBC law that can be implemented on a network of wireless control units utilizing MR dampers. This implementation environment, with limited communication and computational capacity, restricts the agents’ ability to be omniscient and to solve the centralized control problem individually.Additionally, the communication rate of wirelessly networked controllers may not be rapid enough to allow for the market to iterate negotiations to quickly converge to a solution. One possible solution to these problems, which serves as the basis of this paper, is to allow the agents to use simple heuristics that permit each agent to approximately increase their objective function by estimating the environment in which they are acting. The formulation of the heuristics, rules, and objective functions will be presented for a single SDOF system with a single buyer and seller. The concepts will then be extended to more complex MDOF systems.

1)The Supplier’s Utility

The goal of the supply agent, representing the MR damper, should be to minimize the amount of actuation supplied, and thus minimize the power consumed. The cost, in amperes, should increase as the specified amount of force increases up to some saturation limit. Unfortunately the force saturation limit of MR dampers is a dynamic property that changes with respect to the damper’s velocity and hysteresis. For a supply agent to compute the current saturation limit it must employ a model of the nonlinear dynamics of the MR damper. While many models of MR dampers have been proposed, the supply agent will use a Bouc-Wen hysteretic model[16]. The Bouc-Wen model was first applied to model MR dampers by Spencer et al. [17] then adapted for use in real time systems by Lin [18]. The nonlinear force velocity relationship of the MR damper is defined as

where C(V) is the viscous damping coefficient as a function of command voltage,V,y(t) is the differential displacement between the body of the MR damper and its shaft, F is the force in the damper, andz(t) is the hysteresis force of order nas defined by (7).

The Euler method of integration is applied to (7) to compute a discrete time equation of the 2rd order hysteretic force, (8), that the supply agents can exploit at each time step.

Before the model could be codified, the voltage dependent parameters C, A, β, and γ were identified for 10 different voltage level using the methods in [18] and then stored in a lookup table for use by the agents.

The cost of control force is based on the heuristic that an increase in voltage will increase the magnitude of the command force, up to the force produced by the maximum voltage F(Vmax, k) (abbreviated as Fmax). From microeconomic theory the constraints in (9) are placed on the twice differentiable supplier’s cost function Kj(Fj(k+1)). The supplier’s cost function when defined by (10) quantifies the supplier’s heuristic while also abiding by the constraints in (9) which helps to guarantee at least one Pareto optimal solution exists.

The cost function utilizes a one-step ahead prediction of the maximum possible control force magnitude to find the asymptote of the negative logarithmic relationship between a specified control force Fj(V(k+1), k+1) and the cost Kj. The tuning variable μ is a non-negative real number that the designer chooses to adjust the rate of convergence to the asymptote.The goal of each supplier j is to maximize their profit by producing Fj units of control force at the market price p* that solves

2)The Buyer’s Utility

The goal of the buyer in MR-MBC is to minimize the response of the structure by purchasing control force. In this study, the buyers are the wireless controllers measuring the response of the structure. The heuristic the agent uses to formulate the utility function says that an increase in the magnitude of control force, F(k), should decrease the inter-story drift and velocity. The utility function, Φ, of the buyer as defined by (13) follows microeconomic theory in that it is a twice differentiable function bounded by the constraints in (12).

The amount of utility an agent receives from each unit of control force, F, increases as the inter-story drift orvelocity increases. An increase in the buyer’s wealth,w(k), increases the utility received from each unit of control purchased. T, Q, and τ are non-negative constants that the control engineer can choose to adjust how much y(k), ẏ(k), and w(k) affect the agents utility.

The buyer’s utility function is designed to reduce the risk of structural damage (the inter-story drift term) and increase energy dissipation (the inter-story velocity term). It should be noted that the utility function is only based on a heuristic that an increase in control force magnitude will decrease these metrics; clearly, this may not hold for all possible system states. This inaccuracy has the advantages that the utility function is not based on a model (with possible modeling error) and is simple enough that agents can easily compute their own utility. Additionally, the simplicity of the utility function allows for it to be described at any point in time with only two values, (T |y(k)| + Q |ẏ(k)|) and (τ w(k)). This will aid the system in finding the market equilibrium.

3)Equilibrium

With the rules for the agents to formulate their utility functions set, the rules for the agents to communicate and agree on an equilibrium price must be specified.The market’s Pareto optimal equilibrium point occurs when each agent cannot increase their own utility without also decreasing the utility of some other agent. The market achieves this equilibrium by establishing an equilibrium market price that is the only price that force can be traded. Just as in physics, equilibrium is the state of a system where opposing forces are balanced. In the case of markets, the forces are the buyer’s push to make the prices lower, to increase their utility, and the supplier’s push to raise the market price to increase revenue. Due to the physical constraints on the system all of the control force produced must be equal to the control force consumed (e.g. it is impossible for a damper to deliver a force without some part of the structure receiving that force). With these two constraints (psupply = pdemandand Fsupply = Fdemand) the equilibrium must occur where the supply and demand curves intersect. The supply and demand curves intersect at the solution of the optimization problem presented in (14).

The utility function of the buying agents was conveniently formulated such that the first maximization in (14) is the maximization of a 2nd-order polynomial that is strictly concave on the interval 0≤F∞.Therefore the maximum is located where the derivative of the function being maximized is zero. Similarly, the second maximization in (14) is also strictly concaveon the interval and the maximization can be found where the second derivative is zero. These two observations lead to the following simplification in finding Feq and peq*.

A solution must exist to the two simultaneous equations in (15)for F ≥ 0 due to constraints (9) and (12). Therefore the Pareto optimal solution for the SDOF case of the MR-MBC is guaranteed and occurs when an equilibrium quantity (16) is traded at a price determined by (15) with F = Feq.Fig. 2shows that this equilibrium exists at the intersection of the supply and demand curves for some arbitrary state.